The invention pertains to a drive unit for adjusting window panes, sunroofs, seats and other motor vehicle parts.
Systems for adjusting window panes, sunroofs, seats and the like in motor vehicles should be constructed such that the motor vehicle part cannot be forcibly adjusted and does not adjust on its own. Thus the system must be constructed in such a way that it does not permit power transmission in a reverse direction. In a scissor-type window lifter this can at least be partially effected in that the scissor occupies a position close to its dead center. In other systems, for example in a cable-operated wndow lifter, other solutions have to be used.
One known drive unit is based on a self-locking worm gear. By using a self-locking worm gear it is assumed that the drive unit cannot be driven from the output side and accordingly automatic displacement of a window pane, a sunroof or a seat would not be possible. This would be so for worm gears mounted on stationary racks. This, however, is not so for worm gears mounted in motor vehicles, because the continuous vibrations and concussions result in movement of the worm gear from the output side. The movement is increased by the flank clearance always appearing between worm wheel and worm shaft. In particular, this occurs where the part to be adjusted contributes to this undesired action with its own weight.
The drive unit of German specification OS No. 1 708 310 utilizes a reversible electric motor behind which is inserted a worm gear with a worm shaft and a worm wheel meshing with the worm. The blocking function of the gear is available even if the worm gear is not entirely self-locking. The worm shaft is mounted such that it is axially adjustable within limits and rests against a stop which resiliently gives way in the axial direction. A brake is associated with the worm shaft. The brake impedes rotation of the worm if, during a reversal of the power transmission from the worm wheel to the worm, the axial thrust overcomes the spring force of the stop and displaces the worm axially. During reversal of the transmission of power, this axial thrust is always greater than the spring force of the stop, so that the brake becomes always effective and the element to be adjusted cannot be moved. The brake includes a friction disk aligned perpendicularly to and mounted on the worm shaft. The disk co-operates with a friction surface formed on a stationary part of the drive unit, when the worm shaft is axially displaced in one direction. Thus the worm shaft is coupled with the stationary part of the unit. This stationary part is thus the brake element.
If the worm shaft of a system is axially displaceable, the resilient stop is necessary to prevent braking of the worm shaft when the window is closed from the drive side. Of course the force of the resilient stop is also effective when the window is opened from the drive side. Then it adds to the force already acting on the worm shaft because of the worm wheel resisting its rotation. This increased axial thrust diminishes the efficiency of the unit and results in increased wear and noise.